1. Field of the Invention
This invention relates generally to air conditioning systems and more particularly to a control valve for refrigerant entering an evaporator of an automotive air conditioning system.
It is conventional in automotive air conditioning systems to use a control valve to regulate the flow of refrigerant into the evaporator. The control valve throttles flow of the refrigerant liquid passing from the condenser into the evaporator to thereby reduce the pressure of the refrigerant entering the evaporator and provide efficient operation of the system.
One type of control valve currently in use is an expansion valve which senses evaporator pressure and temperature and regulates refrigerant flow by moving a pin along its longitudinal axis to displace a valve element from a valve seat disposed in the flow stream against the bias of a spring. The position of the valve element determines the degree which the refrigerant liquid passing from the condenser into the evaporator is throttled. Such valves are effective in maintaining reasonably optimum system performance under varying operating conditions; however, they are relatively expensive and complex mechanisms which are subject to malfunction requiring costly replacement expenses. Another disadvantage of such valves relates to the difficulty in providing sufficient space to fit the relatively large package in many engine compartments.
Another common valve currently in use, known as an orifice tube, comprises a short tube having a relatively small diameter bore which throttles flow of the refrigerant liquid. Although the ideal size of the orifice varies with the particular operating conditions of the system; a compromise is made in which the diameter of the tube is generally selected so that under conditions which include road speeds and relatively low temperature air the orifice throttles flow to provide efficient operation of the system. However, at idle speeds and high ambient temperature conditions system refrigerant pressure rises causing a concomitant increase in evaporator pressure. This results in increasing the refrigerant's saturation temperature and decreasing the cooling efficiency of the system. Under such conditions, a smaller diameter orifice would restrict flow to a greater extent and thereby lower evaporator pressure and improve the cooling efficiency of the system.
Attempts have been made using a tube having an orifice diameter selected to provide suitable flow at normal road speed conditions along with means to change the effective size of the orifice to restrict flow and decrease pressure downstream of the control valve at operating conditions which increase the condenser pressure, such as high ambient temperature, idle speed conditions. An example of a device incorporating such flow restricting means is shown in U.S. Pat. No. 4,951,478 in which an orifice tube is supported in a piston movably mounted in a cylinder. The orifice and piston in one position functions in the same manner as a conventional fixed orifice control; however increased pressure causes the piston to move against the bias of a spring to a second position adjacent a solid plug axially aligned with the exit end of the orifice tube to restrict flow through the tube. While this type of device could be effective in providing restricted flow when condenser pressure increases, the components are very small making it relatively costly to manufacture and assemble.